This is the largest "sigh" a human can produce. Usually associated with disdain or disgust , misery and woe, or a women on her period. It comes from the comparison of the large exhaling released by a hippopotamus during the breathing process.

"Oh man i have that paper to do.....SIGHAPOTAMUS!"
"Yo dude my girl just told me shes on the rag, again! SIGHAPOTAMUS!"
"The police are here....AGAIN! SIGHAPOTAMUS!"